Heat-activatedly bondable sheetlike elements (heat-activatable sheetlike elements) are used in order to obtain high-strength connections between adherends. Especially suitable are sheetlike elements of this kind for achieving, in the case of a relatively thin bondline, strengths comparable with or higher than those possible with sheetlike elements which contain exclusively pressure-sensitive adhesive systems. Such high-strength bonds are important particularly in light of the ongoing miniaturization of electronic devices, in the consumer electronics, entertainment electronics or communications electronics segment, for instance, as for example for cell phones, PDAs, laptops and other computers, digital cameras, and display devices such as displays and digital readers, for instance.
The requirements in terms of processability and stability of adhesive bonds are increasing particularly in portable consumer electronics articles. One reason for this is that the dimensions of such articles are becoming ever smaller, and so the area that can be utilized for an adhesive bond is also reduced. Another reason is that an adhesive bond in such devices must be particularly stable, since portable articles are required to withstand severe mechanical loads such as impacts or drops, for instance, and, moreover, are to be used across a broad temperature range.
In products of these kinds, therefore, it is preferred to use heat-activatedly bondable sheetlike elements which have heat-activatedly bonding adhesives, i.e., adhesives which at room temperature have no inherent tack, or at best a slight inherent tack, but which, when exposed to heat, develop the bond strength needed for a bond to the respective bonding substrates (adherends, adhesion base). At room temperature, heat-activatedly bonding adhesives of these kinds are frequently in solid form, but in the course of bonding, as a result of temperature exposure, are converted either reversibly or irreversibly into a state of high bond strength. Reversibly heat-activatedly bonding adhesives are, for example, adhesives based on thermoplastic polymers, whereas irreversibly heat-activatedly bonding adhesives are, for instance, reactive adhesives, in which thermal activation triggers chemical reactions such as crosslinking reactions, for example, thereby making these adhesives particularly suitable for permanent high-strength bonds.
In this context there is a requirement more particularly for increasingly thin adhesive tapes, with no reduction in the strength requirements. Heat-activatable films are presently available in a very wide thickness range—thus thicknesses of 30 to 250 μm are not unusual.
A feature common to all heat-activatedly bonding adhesive systems is that for bonding they must be heated. Particularly in the case of bonds where the adhesive systems are hidden from the outside over their full area by the bonding substrates, it is particularly important for the heat necessary for melting or for activating the adhesive to be transported toward the bonding area quickly. If one of the bonding substrates here is a good thermal conductor, then it is possible to heat that bonding substrate by means of an external heat source, as for example through a direct heat transfer medium, an infrared heater or the like.
In the case of such direct heating or contact heating, however, the short heating time that is needed for rapid, homogeneous heating of the known adhesive can be realized only for a large temperature gradient between the heat source and the bonding substrate. Consequently, the bonding substrate that is to be heated ought itself to be insensitive with respect to temperatures which in some cases may even be considerably higher than actually necessary for the melting or activating of the adhesive. Accordingly, the use of heat-activatable adhesive films is problematic for plastic/plastic bonds. Plastics used in particular in consumer electronics include, for example, polyvinyl chloride (PVC), acrylonitrile-butadiene-styrene copolymers (ABS), polycarbonates (PC), polypropylene (PP) or blends based on these plastics.
The situation is different, then, if none of the bonding substrates is a sufficiently good thermal conductor or if the bonding substrates are sensitive toward higher temperatures, as is the case, for example, with many plastics, but also with electronic components such as semiconductor components or liquid-crystal modules, for instance. For the bonding of bonding substrates made from materials of low thermal conductivity or from heat-sensitive materials, therefore, it is appropriate to equip the heat-activatedly bondable sheetlike element itself with an intrinsic mechanism for heating, so that the heat required for bonding need not be introduced from the outside, but is instead generated directly in the interior of the sheetlike element itself. In the prior art there are various mechanisms known that allow such internal heating to be realized, in the form, for instance, of heating by means of an electrical resistance heater, through magnetic induction or by interaction with microwave radiation.
Heating in an alternating magnetic field is achieved on the one hand through induced eddy currents in electrically conductive receptors and on the other hand—to give a model-based explanation—through hysteresis losses by the surrounding elementary magnets in the alternating field. For eddy currents to develop, however, the conductive domains are required to have a certain minimum size. The lower the frequency of the alternating field, the greater this minimum size is. Depending on the receptor material, both effects occur in unison (e.g., magnetic metals) or only one effect occurs in each case (e.g., eddy currents only in the case of aluminum; hysteresis only in the case of iron oxide particles).
In principle, a variety of heating devices for inductive heating are known; one of the parameters which can be used to distinguish them is the frequencies possessed by the alternating magnetic field generated using the heating device in question. For instance, induction heating may be accomplished using a magnetic field whose frequency is situated in the frequency range from about 100 Hz to about 200 kHz (the so-called medium frequencies; MF) or else in the frequency range from about 300 kHz to about 100 MHz (the so-called high frequencies; HF). In addition, as a special case, there are also heating devices known whose magnetic field possess a frequency from the microwave range, such as the standard microwave frequency of 2.45 GHz, for example.
Rising in line with the frequency of the alternating field used is the technical cost and complexity involved in generating the alternating field, and hence the costs of the heating device. Whereas middle-frequency systems are already currently available at a market price of around 5000 euros, the outlay for high-frequency systems is at least 25 000 euros. Also rising in line with the frequency, furthermore, are the safety requirements concerning the heating system, and so, for high-frequency systems, it is regularly necessary to add, to the higher acquisition costs, higher costs for the installation of such technology as well.
Where high frequencies are used for the adhesive bonding of components in electronic devices, it is possible, furthermore, for unwanted damage to occur to electronic components in these devices in the course of their exposure to the alternating electromagnetic field.
Example applications that may be given for induction heating include manufacturing operations from the areas of bonding, seam sealing, curing, tempering, and the like. The usual technique here is to employ those methods where the inductors surround components completely or partially and heat them uniformly over the entire extent or, when required, deliberately nonuniformly, in accordance, for example, with EP 1 056 312 A2 or with DE 20 2007 003 450 U1.
DE 20 2007 003 450 U1 sets out for example, inter alia, a method for fusing a container opening with a sealing film, in which the metallic inlay of a sealing film is heated by induction and a sealing adhesive is melted by conduction of heat. The containers are closed with a screw-on or snap-in lid, which comprises a metal foil and an adjacent polymeric sealing film. Using the induction coil, eddy currents are generated in the metal foil, and heat the metal foil. As a result of the contact between metal foil and sealing film, the sealing film is also heated and is thereby fused with the container opening. Induction coils in tunnel form have the advantage over flat coils that they can be used as well to seal containers having a large distance between the metal foil and the top edge of the lid, since the coil acts on the metal foil from the side.
A disadvantage of this method is that a substantially greater part of the component volume than the pure adhesive volume and the metal foil is passed through the electromagnetic field and hence, in the case of an electronic component, instances of damage are not ruled out, since heating may occur at unwanted locations. A further disadvantage is that the entire lid film is heated, whereas only the edge region in contact with the container would be sufficient for bonding. Hence there is a large ratio of heated area to bonding area, which for typical beverage bottles having an opening diameter of 25 mm and a bonding width of 2 mm is approximately 6.5. For larger container diameters, the ratio goes up in the case of the usually constant bonding width.
In recent years, for the inductive heating particularly in the bonding of plastic on plastic, inductively heatable heat-activatable adhesive films (HAFs) have moved back beneath the spotlight. The reason for this is to be found in the nanoparticulate systems that are now available, such as MagSilica™ (Evonik AG), for example, which can be incorporated into the material of the body to be heated and which thus allow heating of the body throughout its volume, without any attendant significant detriment to its mechanical stability.
Because of the small size of these nanoscopic systems, however, it is not possible to bring about efficient heating of such products in alternating magnetic fields with frequencies from the medium frequency range. For the innovative systems, instead, frequencies from the high frequency range are required. It is at these frequencies in particular, however, that the problem of damage to electronic components in an alternating magnetic field is manifested to a particularly severe extent. Generating alternating magnetic fields with frequencies in the high frequency range, moreover, requires increased cost and complexity of apparatus, and is therefore unfavorable economically. Furthermore, the use of nanoparticulate fillers is a problem from the standpoint of the environment as well, since these fillers are not easily separated from the surrounding materials on subsequent recycling. It is difficult, furthermore, to use these particles in very thin films, since the strong tendency of nanoparticulate systems to form agglomerates means that the films produced therewith are usually very inhomogeneous.
Furthermore, and in order to avoid the above problems, it is possible for heat-activatable films (HAFs) which are intended to be inductively heatable to be filled with sheetlike metallic or metallized structures. This is very efficient in the context of the use of full-area metal foils, even in the medium frequency range; high heating rates can be achieved, and so induction times of between 0.05 and 10 s can be realized. Also possible in this context is the use of very thin conductive films of between 0.25 μm and 75 μm.
Also known is the use of perforated metal foils, wire meshes, expanded metal, metal webs or fibers, through which the matrix material of the HAF is able to penetrate, thereby improving the cohesion of the assembly. The efficiency of heating, however, goes down as a result.
For adhesive bonds within mobile electronic devices, the product Duolplocoll RCD from Lohmann is known, this product being equipped with inductively heatable nanoparticles. This product can be heated in a technically utilizable way exclusively in the high frequency range. The disadvantages described above with the use of particles and high-frequency alternating fields apply to this product as well.